[en] For miniaturized capacitive energy storage, volumetric and areal capacitances are more important metrics than gravimetric ones because of the constraints imposed by device volume and chip area. Typically used in commercial supercapacitors, porous carbons, although they provide a stable and reliable performance, lack volumetric performance because of their inherently low density and moderate capacitances. In this paper, we report a high-performing electrode based on conductive hexaaminobenzene (HAB)-derived two-dimensional metal-organic frameworks (MOFs). In addition to possessing a high packing density and hierarchical porous structure, these MOFs also exhibit excellent chemical stability in both acidic and basic aqueous solutions, which is in sharp contrast to conventional MOFs. Submillimetre-thick pellets of HAB MOFs showed high volumetric capacitances up to 760 F cm^-3 and high areal capacitances over 20 F cm^-2. Furthermore, the HAB MOF electrodes exhibited highly reversible redox behaviours and good cycling stability with a capacitance retention of 90% after 12,000 cycles. In conclusion, these promising results demonstrate the potential of using redox-active conductive MOFs in energy-storage applications.

[en] NiCo₂O₄ directly grown on nickel foam was fabricated via a simple hydrothermal method in glycol solution with different volume ratios of water to glycol with the aim to control the shape of as-prepared products. It was found that NiCo₂O₄ shape strongly depends on glycol content in reaction solution. As 1:0, 1:1, 1:2 and 1:3 volume ratios of water to glycol were selected, the corresponding NiCo₂O₄ exhibited 3D hierarchical honeycombs composed of numerous nanorods, 1D nanoneedles, 2D interconnected nanosheets and ultrathin nanosheets with numerous micropores, respectively. The morphology of NiCo₂O₄ has remarkable influence on electrochemical performance. The NiCo₂O₄ nanoneedles with open mesoporous structure and ultrathin nanosheets with numerous micropores displayed superior electrochemical properties than other products. The specific capacitances of NiCo₂O₄ nanoneedles and nanosheets can reach 708 and 634 F g⁻¹ at current density of 1 A g⁻¹, respectively. Rate capabilities 88.7% and 92.9% and cycling performance of 86.5% and 88% capacitance retentions at 5 A g⁻¹ after 2000 cycles were obtained. These outstanding results are ascribed to their structure advantages, i.e. open mesoporous feature, numerous electroactive surface sites and excellent conductivity, guaranteeing short ions diffusion path and fast faradic reaction rate. (paper)

[en] Highlights: • Ni_xP nanowires/Ni hybrid foam is obtained by a facile and large-scale method. • Supercapacitor performance for Ni_xP nanowires/Ni hybrid foam is investigated. • Ultrahigh rate performance is observed for Ni_xP nanowires/Ni hybrid foam. • Ni_xP nanowires/Ni hybrid foam shows long-term cycling stability.

[en] Highlights: • Successful synthesis of poly(vinyl alcohol), PVA, and graphene oxide composite. • Improvement of conductivity of poly(vinyl alcohol) with reduced graphene oxide. • Improvement of electrochemical properties of PVA with reduced graphene oxide. Poly(vinyl alcohol), PVA, polymer was successfully combined with graphene oxide (GO) and thermally reduced graphene oxide (RGO), respectively, to make composites and characterized for supercapacitor applications. PVA-RGO composite shows excellent electrochemical properties compared to PVA-GO composite. The capacitance of 190 Fg⁻¹ is obtained from PVA-RGO composite which is larger than that (13 Fg⁻¹) of PVA-GO composite. Electrochemical impedance of PVA-RGO is more than ten times smaller than that of PVA-GO at 20 kHz, demonstrating that PVA-RGO composite has a great advantage for supercapacitor applications compared to PVA, GO, RGO, and PVA-GO composite.

[en] Flexible supercapacitors with electrodes coated on inexpensive fabrics by the dipping technique. This paper present details of the design, fabrication and characterisation of fabric supercapacitor. The sandwich structured supercapacitors can achieve specific capacitances of 11.1F/g, area capacitance 105 mF.cm⁻² and maintain 95% of the initial capacitance after cycling the device for more than 15000 times

[en] Highlights: • The performances degradation of supercapacitors during power cycling ageing tests have been quantified. • The performances recovery phenomena of supercapacitors is highlighted and modelled. • The impact of the rest conditions (temperature and cut-off voltage) on the recovery behaviour is quantified. • An accurate ageing model able to predict the performances degradation of supercapacitors in power cycling is proposed. - Abstract: During accelerated ageing tests of supercapacitors (SC), a decay in their performance is reflected by a decrease in capacitance and an increase in equivalent series resistance ESR. In power cycling, when electric solicitations of the SC are interrupted for the purposes of real use or characterisation, performance recovery is observed, mainly in terms of an increase in capacitance. This phenomenon is due to a redistribution of electrical charges, balancing of impurities inside the porous carbon electrodes, and the cell’s return to thermodynamically steady-state conditions. A repetitive long rest period during cycling appears to slow down the ageing process, and to reduce the decay in performance. The impacts on capacitance recovery during rest time, of both cut-off voltage and temperature, are studied. A nonlinear analytical expression is used to predict the capacitance decay for several durations and test interruption periodicities; this is also used to model the capacitance during rest time, taking the cut-off voltage, rest time and temperature into account

[en] PIn/MoS₂ composite prepared via in-situ oxidative polymerization of indole monomer in the presence of exfoliated MoS₂ sheets shows fair capacitive performance. The specific capacitance reaches a value of 173 F g^-1 at the current density of 1 A g^-1. The close integration of PIn and MOS₂ generating interfaces involving two different materials leads to enhanced charge storage properties in the composite. The composite stores charge in bulk mainly through pseudocapacitive mechanism along with some surface storage. The presence of micro and mesopores enhances the surface area and also facilitates the easy charge transport through the electrode material. The composite also shows excellent cyclic stability due to the presence of stable MoS₂. (author)

[en] Highlights: • The specific capacitance is raised to 307 Fg^-1 with 30 wt% Fe₂O₃ addition. • The system retains 92% of its initial capacity upon 200 cycles. • High specific energy of 24.9 Wh kg⁻¹ and specific power of 722.2 kW kg⁻¹ is attained. • The pseudocapacitive contribution enhanced the overall storage capacity.

[en] Highlights: • Current-voltage reciprocity of an R_s-CPE-equivalent supercapacitor is verified. • The accumulated charge on a supercapacitor depends on the charging waveform. • Effective capacitances due to voltage and current charging are derived and compared. • Voltage-charging of a supercapacitor is more efficient than current-charging. - Abstract: The focus in supercapacitor research typically falls into one of two categories: (i) the rational design and engineering of electrode materials and electrolyte formulation to achieve high performance devices at competitive costs, and (ii) the modeling of their resulting behavior in response to constant-current charging/discharging, cyclic voltammetry or impedance spectroscopy. However, less work has been dedicated to new ways for charging these devices. In this work we show that charging a supercapacitor, modeled as a constant phase element with a series resistor, using a linear voltage ramp results in higher stored charge and higher effective capacitance value than when using a linear current ramp. This is despite the reciprocity of the device, as we proved analytically. The theoretical analysis and numerical simulations are in excellent agreement with the experimental results carried out on a commercial supercapacitor. The findings can be viewed as a step towards finding the optimum charging waveforms for these devices that would maximize their effective capacitance.

No abstract available